Field evidences of the nonlinear resonant coupling between two adjacent basins are found in two elongated inlets in the region of Ciutadella, Menorca Island, in the western Mediterranean. Sea level measurements reveal different amplification responses during calm periods (when a linear approach is justified) and during rissaga events (when sea level oscillations inside the bays reach amplitudes comparable to water depth and nonlinear effects are expected to play a significant role). These differences are then interpreted as the observational manifestation of the nonlinear coupling between two adjacent inlets. This phenomenon is studied using a simplified analytical model consisting of two parallel and rectangular adjacent inlets with constant depth. Weakly nonlinear and weakly dispersive waves are considered. Therefore Boussinesq equations may be employed for the analysis. The problem is further simplified by assuming that the waves outside the bays are linear so that the wave fields inside the bays can be described by a system of coupled nonlinear two-point boundary value problems with complex coefficients for each harmonic. The nonlinear system is solved numerically, and the solutions are used to investigate the coupling under linear and nonlinear wave conditions.